The present invention relates to an apparatus and a method for image signal processing that shift the position of detected pixels of an image signal generated by double speed conversion in which signal one frame is formed by two fields or four fields.
As a scanning system of television broadcast, the interlaced scanning system has conventionally been most widely used in which every other horizontal scanning line is scanned in an interlaced manner. The interlaced scanning system forms one frame image of a field image formed by odd-numbered scanning lines and a field image formed by even-numbered scanning lines. The interlaced scanning system thereby suppresses plane flicker disturbance in which the whole screen appears to be flickering, and thus prevents degradation in picture quality.
The interlaced scanning system is employed as a television standard in various countries in the world. The PAL (Phase Alternation by Line) system used in television broadcast in Europe, for example, has a field frequency of 50 [Hz] (25 frame images per second and 50 field images per second).
In order to further suppress plane flicker disturbance, in the PAL system, in particular, a field frequency doubling method has conventionally been employed which converts an input image signal having the field frequency of 50 Hz into an image signal having double the frequency of 100 Hz by performing processing such as interpolation or the like.
FIG. 18 shows an example of block configuration of a field double speed conversion circuit 5 to which the field frequency doubling method is applied. The field double speed conversion circuit 5 is integrated into a television receiver 6 that includes an input terminal 61, a CRT 63, and a horizontal and vertical deflection circuit 62. The field double speed conversion circuit 5 includes a double speed conversion unit 51 and a frame memory 52.
The double speed conversion unit 51 writes an image signal of 50 fields per second of the PAL system, for example, inputted from the input terminal 61 into the frame memory 52. Also, the double speed conversion unit 51 reads the image signal written into the frame memory 52 at a speed twice that at the time of the writing. The double speed conversion unit 51 can thereby double the frequency of the image signal of 50 fields per second, and generate an image signal of 100 fields per second.
The double speed conversion unit 51 outputs the double-speed-converted image signal to the CRT 63. The CRT 63 displays the image signal inputted thereto on the screen. Deflection in a horizontal and a vertical direction for the image signal in the CRT 63 is controlled on the basis of horizontal and vertical sawtooth waves having a frequency twice that of the input image signal and generated by the horizontal and vertical deflection circuit 62.
FIGS. 19A and 19B show a relation between a pixel position and each field in image signals before and after double speed conversion. In the figure, the axis of abscissas indicates time, and the axis of ordinates indicates pixel position in a vertical direction. The image signal indicated by white circles in FIG. 19A is an interlaced image signal of 50 fields per second before the double speed conversion, while the image signal indicated by black circles in FIG. 19B is an interlaced image signal of 100 fields per second after the double speed conversion.
In the image signal shown in FIG. 19A, a field f1 and a field f2 are signals generated from the same film frame, and similarly a field f3 and a field f4 include the same film frame. Since these image signals are interlaced image signals, the pixel position in the vertical direction differs between fields adjacent to each other. Hence, it is not possible to newly generate one field between each pair of fields while maintaining the interlaced state.
Accordingly, as shown in FIG. 19B, two fields f2′ and f1′ are newly generated between the field f1 and the field f2. No fields are generated between the field f2 and the field f3. Two fields f4′ and f3′ are newly generated between the field f3 and the field f4. Thus, one film frame is formed by four fields or two frames.
A value of each pixel of the newly generated fields f1′, f2′, . . . may be obtained as a median value of three pixels around the periphery of the pixel by using a median filter or the like. The newly generated fields f1′, f2′, . . . have the same contents as the fields f1, f2, . . . , respectively.
Thus, the field double speed conversion circuit 5 alternately disposes a portion where two fields are newly generated and a portion where no fields are generated between fields of the image signal before the double speed conversion. It is thereby possible to increase the number of screens per unit time and consequently prevent the above-mentioned plane flicker disturbance.
For a cinema film formed by still pictures of 24 frames per second to be viewed on ordinary television, television-cinema conversion (hereinafter referred to as telecine conversion) is performed to convert the film into an interlaced television signal. FIGS. 20A and 20B show a relation between an image position and each field when an image of the image signal after the telecine conversion moves in a horizontal direction. In the figures, the axis of abscissas indicates image position in the horizontal direction, and the axis of ordinates indicates time. Since the fields f1 and f2 of the image signal before double speed conversion shown in FIG. 20A include the same film frame, the image is displayed at the same position in the fields f1 and f2. With a transition to the field f3, the image moves in the horizontal direction (to the right). Since the field f4 and the field f3 form the same film frame, the image in the field f4 is displayed at the same position as in the field f3.
After the image signal after the telecine conversion shown in FIG. 20A is subjected to double speed conversion by the field frequency doubling method, the same image is displayed at the same position in the fields f1, f2′, f1′, and f2 forming the same film frame, as shown in FIG. 20B. Similarly, the same image is displayed at the same position in the fields f3, f4′, f3′, and f4 forming the same film frame.
FIG. 21A shows a relation between an image position and each field when an image of a television signal (hereinafter referred to as a TV signal) before double speed conversion moves in the horizontal direction. In FIG. 21A, fields f1, f2, f3 . . . each form an independent film frame, and therefore the image is displayed at different positions in the fields. The image moves in the horizontal direction (to the right) with each transition from the field f1 to f2, f3 . . . .
After the image signal of the TV signal shown in FIG. 21A is subjected to double speed conversion by the field frequency doubling method, the same image is displayed at the same position in the fields f1 and f2′ forming the same film frame, as shown in FIG. 21B. Similarly, the same image is displayed at the same position in the fields f1′ and f2 forming the same film frame.
However, as shown in FIG. 20B, while the image of the image signal after the telecine conversion and the double speed conversion is displayed at the same position in the fields f1 to f2, the image moves greatly in the horizontal direction when a transition is made from f2 to f3. Similarly, as shown in FIG. 21B, while the image of the image signal obtained by subjecting the TV signal to the double speed conversion is displayed at the same position in the fields f1 and f2′, the image moves greatly in the horizontal direction when a transition is made from f2′ to f1′.
In particular, the output image signal forms each field at a regular interval of 1/100 second. Therefore, a time period of motion of the image is shorter than a time period of stillness of the image. When a program is actually viewed on the CRT, motion of the image appears to be discontinuous.
Further, it is necessary to efficiently eliminate the discontinuity of image motion even when both a telecine-converted image signal and a TV signal are inputted.